The present invention generally relates to apparatuses and methods to perform rotary steerable directional drilling operations. More particularly, the present invention relates to downhole actuators to position a drill bit assembly in a desired trajectory by a rotary steerable assembly. More particularly still, the present invention relates to a bi-directional actuator to be used in a rotary steerable system to accommodate more precise positioning of a drill bit assembly.
Boreholes are frequently drilled into the Earth's formation to recover deposits of hydrocarbons and other desirable materials trapped beneath the Earth's crust. Traditionally, a well is drilled using a drill bit attached to the lower end of what is known in the art as a drillstring. The drillstring is a long string of sections of drill pipe that are connected together end-to-end through rotary threaded pipe connections. The drillstring is rotated by a drilling rig at the surface thereby rotating the attached drill bit. The weight of the drillstring typically provides all the force necessary to drive the drill bit deeper, but weight may be added (or taken up) at the surface, if necessary. Drilling fluid, or mud, is typically pumped down through the bore of the drillstring and exits through ports at the drill bit. The drilling fluid acts both lubricate and cool the drill bit as well as to carry cuttings back to the surface. Typically, drilling mud is pumped from the surface to the drill bit through the bore of the drillstring, and is allowed to return with the cuttings through the annulus formed between the drillstring and the drilled borehole wall. At the surface, the drilling fluid is filtered to remove the cuttings and is often used recycled.
In typical drilling operations, a drilling rig and rotary table are used to rotate a drillstring to drill a borehole through the subterranean formations that may contain oil and gas deposits. At downhole end of the drillstring is a collection of drilling tools and measurement devices commonly known as a Bottom Hole Assembly (BHA). Typically, the BHA includes the drill bit, any directional or formation measurement tools, deviated drilling mechanisms, mud motors, and weight collars that are used in the drilling operation. A measurement while drilling (MWD) or logging while drilling (LWD) collar is often positioned just above the drill bit to take measurements relating to the properties of the formation as borehole is being drilled. Measurements recorded from MWD and LWD systems may be transmitted to the surface in real-time using a variety of methods known to those skilled in the art. Once received, these measurements will enable those at the surface to make decisions concerning the drilling operation. For the purposes of this application, the term MWD is used to refer either to an MWD (sometimes called a directional) system or an LWD (sometimes called a formation evaluation) system. Those having ordinary skill in the art will realize that there are differences between these two types of systems, but the differences are not germane to the embodiments of the invention.
A popular form of drilling is called “directional drilling.” Directional drilling is the intentional deviation of the wellbore from the path it would naturally take. In other words, directional drilling is the steering of the drill string so that it travels in a desired direction. Directional drilling is advantageous offshore because it enables several wells to be drilled from a single platform. Directional drilling also enables horizontal drilling through a reservoir. Horizontal drilling enables a longer length of the wellbore to traverse the reservoir, which increases the production rate from the well. A directional drilling system may also be beneficial in situations where a vertical wellbore is desired. Often the drill bit will veer off of a planned drilling trajectory because of the unpredictable nature of the formations being penetrated or the varying forces that the drill bit experiences. When such a deviation occurs, a directional drilling system may be used to put the drill bit back on course.
A traditional method of directional drilling uses a bottom hole assembly that includes a bent housing and a mud motor. The bent housing includes an upper section and a lower section that are formed on the same section of drill pipe, but are separated by a permanent bend in the pipe. Instead of rotating the drillstring from the surface, the drill bit in a bent housing drilling apparatus is pointed in the desired drilling direction, and the drill bit is rotated by a mud motor located in the BHA. A mud motor converts some of the energy of the mud flowing down through the drill pipe into a rotational motion that drives the drill bit. Thus, buy maintaining the bent housing at the same azimuth relative to the borehole, the drill bit will drill in a desired direction. When straight drilling is desired, the entire drill string, including the bent housing, is rotated from the surface. The drill bit angulates with the bent housing and drills a slightly overbore, but straight, borehole.
A more modern approach to directional drilling involves the use of a rotary steerable system (RSS). In an RSS, the drill string is rotated from the surface and downhole devices force the drill bit to drill in the desired direction. Rotating the drill string is preferable because it greatly reduces the potential for getting the drillstring stuck in the borehole. Generally, there are two types of RSS, “point the bit” systems and “push the bit” systems. In a point system, the drill bit is pointed in the desired position of the borehole deviation in a similar manner to that of a bent housing system. In a push system, devices on the BHA push the drill bit laterally in the direction of the desired borehole deviation by pressing on the borehole wall.
A point the bit system works in a similar manner to a bent housing because a point system typically includes a mechanism to provide a drill bit alignment that is different from the drill string axis. The primary differences are that a bent housing has a permanent bend at a fixed angle and a point the bit RSS typically has an adjustable bend angle that is controlled independent of the rotation from the surface. A point RSS typically has a drill collar and a drill bit shaft. The drill collar typically includes an internal orienting and control mechanism that counter rotates relative to the rotation of the drillstring. This internal mechanism controls the angular orientation of the drill bit shaft relative to the borehole. The angle between the drill bit shaft and the drill collar may be selectively controlled, but a typical angle is less than 2 degrees. The counter rotating mechanism rotates in the opposite direction of the drill string rotation. Typically, the counter rotation occurs at the same speed as the drill string rotation so that the counter-rotating section maintains the same angular position relative to the inside of the borehole. Because the counter rotating section does not rotate with respect to the borehole, it is often called “geo-stationary” by those skilled in the art.
A push the bit RSS system typically uses either an internal or an external counter-rotation stabilizer. The counter rotation stabilizer remains at a fixed angle (geo-stationary) with respect to the borehole while the drillstring above is rotated. When borehole deviation is desired, an actuator presses a pad against the borehole wall in the direction opposite the desired trajectory. This operation results in a drill bit that is pushed in a desired direction. Typically, one or more actuator pads are located on a geo-stationary counter-rotating collar of the push the bit apparatus.
Historically, push the bit and point the bit rotary steerable systems use their geostationary components either to aim, or to force the drill bit in a desired direction. When subterranean formations are either unknown or especially treacherous, forcing the bit is not always feasible. In those circumstances, aiming the bit may be preferable to forcing the bit in a wrong direction. Because uncertainty of the formation is always an issue in subterranean drilling, a system having the capabilities of both point and push the bit rotary steerable systems is desirable.